Recent work suggests a prominent role for adipose inflammation in diet induced obesity (DIO) and its metabolic and cardiovascular complications. For example, CCR2, a chemokine receptor, has been implicated in inflammatory monocyte recruitment into obese adipose and into neointima of atherosclerosis. Fractalkine (CX3CL1) and its monogamous receptor (CX3CR1) are also atherogenic and emerging data show additive roles for CX3CL1-CX3CR1 and CCR2 in atherosclerosis. In preliminary studies, we demonstrate that CX3CL1 is markedly induced in adipose inflammation, adipose CX3CL1 is increased in human and rodent obesity, fatty acids and adipocytokines induce CX3CL1 in adipocytes, monocytes adhere to human adipocytes in a CX3CR1-dependent manner, and CX3CR1 deficiency modulates adipose function and energy homeostasis in high-fat fed mice. Thus, CX3CL1-CX3CR1 appears to be one of very few chemokine pathways implicated in both obesity and atherosclerosis and therefore is the focus of this proposal. Further, unlike other chemokines e.g., CCR2, CX3CL1-CX3CR1 may be a relatively safe therapeutic target in human. We propose that CX3CL1 recruits circulating monocytes, independent of CCR2, driving adipose tissue macrophage (ATM) accumulation and survival, leading to insulin resistance and obesity on high-fat diet. The goal of this proposal, using mice models and human translation, is to determine whether fractalkine (CX3CL1-CX3CR1) modulates adipose inflammation and its metabolic consequences and if this action is independent of CCR2. Aim 1 will characterize effects of CX3CL1 deficiency and disruption of CX3CR1 signaling on diet-induced adipose inflammation, energy and glucose homeostasis, and obesity in mice. Aim 2 will determine how CX3CL1 interacts with CCR2 in diet-induced adipose inflammation, insulin resistance and obesity in mice. Aim 3 will address the hypothesis that functional variation in CX3CR1 will affect adipose inflammation and insulin resistance in humans. We will also utilize adipocytes, derived from mouse embryonic fibroblast, and primary human adipocytes to study the role of CX3CL1 in adipocyte induction of monocyte recruitment, adhesion, activation and survival. These studies will define causality and mechanism while providing the human context for clinical and therapeutic development of CX3CL1-CX3CR1 in DIO and its complications.